Auditory brain-stem (ABP) and somatosensory evoked potentials (SEP) in an animal model of a synaptic lesion: elevated plasma barbiturate levels.

Experiments were conducted to determine whether a consistent pattern of auditory nerve brain-stem evoked synaptic lesion model in cats (elevated levels of the lesion model in cats (elevated levels of the barbiturate thiopental). The ABP in response to low (10/sec) and high (80/sec) stimulus rates was recorded. In order to differentiate between the effects of the elevated drug levels on axonal propagation and on synaptic transmission, the early components of the somatosensory evoked potential (SEP) were also recorded, with particular attention to the first SEP wave, which is solely an axonal event without any intervening synapse. Calculations showed that the effect on synapses was 3.0-9.5 times greater than the effect of the drug on axonal propagation. As the level of barbiturates increased (representing a more severe synaptic lesion), the interpeak latencies of the ABP and the SEP became progressively prolonged, more so than the dependence of the first waves of both the ABP and the SEP on drug level. In general, amplitudes were not affected. At progressively elevated drug levels, higher stimulus repetition rates did not have an increasingly greater effect than lower rates on evoked response latencies and amplitudes so that this study also shows that the use of elevated stimulus rates does not hold much promise in the diagnosis of synaptic lesions.